Assigning new Boot Disk MDD G4 OS 10.4

Hi Åll, I've just purchased another G4 MDD FW800 Dual 1.42 and am installing drives from an old computer. 1 of the drives I installed has all the apps and OS I'd like to run on this system. When I power the system up I get a blue screen with flashing apple faces that alternate with a file symbol with a question mark on it. I assume the computer is asking which is the boot disk. How do I assign the disk I want to be the new boot disk when the computer stops it's startup at this point?
Many thanks!
Kind Regards,
Chris Kinsman

when I do this, I get two symbols on the screen. On the left is a circular arrow (return?) on the right is an arrow pointing to the right (next?). Pressing either of these doesn't get any response from computer. What now?

It came from and is going to a G4 MDD FW800 Dual 1.42. This drive was an original but was then cloned for safety. In the old machine - it was not designated as the boot drive, the clone was - does this make a difference?

on the back of the drive, you'll see a group of 8 pins (4 rows of pairs of pins). you can use these to designate how a drive will be used. usually if you look at the top of the drive, on the label, it will describe various jumper configurations like master, slave, cable select... it's possible that the drive is set to slave and that may be effecting it's ability to be used as a boot drive. change it to either master or cable select.